Pneumatic rubber tires are often prepared with a rubber tread having a cap/base construction. Such constructions are well known. It may be referred to herein as a composite of such two components.
Typically, the outer cap portion of the tread contains the visible tread grooves and lugs, or raised portions, which is designed to contact the ground. The cap portion usually includes such tread configuration and usually extends to a tread depth of just below the grooves of the tread. The rubber for the cap portion is typically compounded to provide good skid resistance, treadwear and rolling resistance.
The rubber base portion of the tread is positioned beneath the cap portion and, being a part of the tread itself, is located between the outer tread cap and an underlying supporting belt or carcass portion of the tire. Such tire construction is well known. The rubber for the base is typically compounded to enhance rolling resistance and durability for the tire. The terms "rubber composition" and "rubber compound" may be used somewhat interchangeable in this description.
Often, a primary purpose for dividing a tread into an outer cap portion and an inner, underlying base portion is to provide a tread base which will reduce the tire's rolling resistance. Otherwise a single composition tread construction might be satisfactory so that the tread is composed of the tread cap throughout.
In one aspect, the cap/base rubber composite may be designed to improve the rolling resistance of the tire without unduly sacrificing its traction (skid resistance) or treadwear. Such often desirable aspect is usually difficult to obtain with a single tread compound because, for example, rolling resistance reduction is typically obtained at the expense of traction and/or treadwear.
In another aspect, it would seem that increasing the thickness (gauge) of the base rubber compound, while maintaining the same overall thickness of the tread, would provide an additional improvement in tire rolling resistance (lower resistance to the rolling of the tire, usually under loaded conditions).
However, it has been observed that increasing the thickness of the base tread rubber can result, during the molding and curing of the tire, in extreme base peaking of the base rubber into the lugs of the cap rubber itself. Thus, elements of the base are caused to extend substantially outward into the cap portion of the tire. This is disadvantageous both because groove cracking in the tread cap may develop and also primarily as the lugs of the cap wear away as the tire is used, the exposed base compound becoming in contact with the road would result in poor traction and treadwear.
A difficulty of such base peaking may become evident as the tire tread wears during use so that the tread cap becomes thinner and the base peaks eventually become exposed and contact the road surface. The resulting exposed tread surface may then not present optimum tread properties to the road surface.
The phenomenon of the base peaking into the lug portion of the cap is largely attributed to the uncured base rubber viscosity often being lower than the viscosity of the cap rubber. Thus, during the molding and curing of the tire under conditions of heat and pressure, the base rubber may have a greater tendency to flow than the cap rubber and, therefore, allow a displacement of the base rubber as the cap rubber is formed into lugs and grooves.
It is, therefore, desirable to increase the viscosity of the uncured base rubber compound while maintaining a satisfactory resilience (rebound value) of the cured base rubber compound.
In one aspect, the viscosity of the uncured base rubber may be readily increased with conventional compounding ingredients, such as, for example, increasing its carbon black content and, optionally, reducing its oil content. However, it is considered that such technique tends to defeat the overall concept of a tread base because it tends to increase hysteresis of the rubber compound which typically results in poorer rolling resistance of the tire.
The use of trans 1,4-polybutadiene has been disclosed for various purposes, including, for example, tire tread rubber compounds and increasing green strength of rubber mixtures (See Japanese Patent Publication Nos. 60-133,036; 62-101,504 and 61-143,453) and U.S. Pat. No. 4,510,291.
Trans 1,4-polybutadiene has been previously considered for use in a tread base rubber composition. However, it is further considered that the use of trans 1,4-polyisoprene is a departure therefrom.
Use of trans 1,4-polybutadiene in a tread base composition was considered to be an improvement because it reduced the peaking of the base rubber into the outer cap rubber of the tire tread during the tire curing operation. A particular advantage of the trans 1,4-polybutadiene was its relatively high softening point (e.g. 50.degree. C.), so that it would not begin to flow as rapidly as other tread rubbers as the temperature of the tire would increase in the tire mold. However, uniquely, the trans 1,4-polybutadiene exhibited a second, and primary, softening point (e.g. 25.degree. C.) which was substantially lower than its higher, minor softening point.
Uniquely, the trans 1,4-polyisoprene has only one softening point (e.g. 60.degree. C.) and, thus, might possibly provide an even greater advantage of use in a tread base rubber composition. It might tend to flow even less than the trans 1,4-polybutadiene and, thus, advantageously even further reduce base tread rubber peaking into the tread cap rubber during curing because it does not have the second, lower, primary softening point (e.g. 25.degree. C.) so that it might not be expected to substantially flow in the mold at or near such lower temperature.
Uniquely, trans 1,4-polyisoprene is typically a thermoplastic resin rather than most other rubbers in its uncured state at room temperature by virtue of its high crystallinity. Because it contains many double bonds in its backbone, it can, however, be suitably blended and co-cured with elastomers to yield a cured rubber composition.